The present invention relates to a method for combining boron into the surface layers of a transition metal. Metals treated in this manner have use in corrosive environments where their untreated substrates may otherwise be severely attacked The method is of particular interest in treating current collectors for use in high temperature electrochemical cells employing molten alkali metal halides as electrolytes. One other important application is the hardening of metal surfaces to resist wear in mechanical parts.
One type of high temperature secondary electrochemical cells contemplated employ chalcogens such as sulfur, oxygen selenium or tellurium as well as their transition metal chalcogenides as positive electrode materials. Of particular interest at present are positive electrodes including the sulfides of iron. The negative electrodes in such cells include alkali metals such as lithium, sodium or potassium or alkaline earth metals such as calcium, magnesium and alloys of these materials as negative electrode reactants. An extremely corrosive environment is produced by these reactants and the molten halide salts that ordinarily are used in these type cells as electrolyte. For instance, a cell such as an FeS.sub.2 /LiCl-KCl/LiAl presents a difficult problem for selection of a current collector material for the positive electrode. At present molybdenum and molybdenum alloys are considered suitable for this application but these materials are quite expensive.
Representative publications illustrating the field of the present invention include the following.
U.S. Pat. No. 4,172,926, Oct. 30, 1979 to Shimotake et al illustrates a high temperature electrochemical cell that could benefit from current collector material prepared by the method of the present invention.
ANL 78-94 "High Performance Batteries for Electric Vehicle Propulsion and Stationary Energy Storage", October 77-September 78, pages 144-147 show the use of iron boride and other boride coatings on current collectors within FeS.sub.2 electrodes in an effort to reduce corrosion. The coating was prepared by a gaseous diffusion process using a pack cementation method.
Epik "Boron and Refractory Borides" edited by V.I. Matkovich, page 597, Springer-Verlag 1977 shows a variety of methods for boronizing metals. The metals are (1) immersed in a molten salt without applied potential, (2) subjected to chemical vapor deposition, (3) subjected to electrolysis in a molten salt bath or (4) packed in a powder including boron containing materials. In each of these methods, temperatures typically of 850.degree.-1000.degree. C. are employed.
Komatsu et al, J. Japan Institute Metals, 38, page 481, (1974) describes the use of a powder pack with KBF.sub.4 and ferroboron for preparing iron boron coatings on iron substrates at temperatures as low as 600.degree. C.
Molybdenum metal is one of the few materials suitable for the positive current collectors of FeS.sub.2 electrodes within high temperature, molten salt electrochemical cells. However, because of the high cost and poor fabricability of molybdenum metal, efforts to find alternative positive current collector materials have been made.
Electronically conductive ceramic coatings such as the borides on metallic substrates have been considered for current collector use. Various methods for producing boron coatings particularly on iron substrates have been attempted. Iron boron coatings have been prepared by electrolysis from a molten alkali metal halide baths at temperatures typically of 900.degree. C. but such coatings have failed to provide good corrosion resistance within a cell environment. Other boride coatings on iron have been prepared by packing a ceramic powder with boron containing materials closely around the substrate and heating to elevated temperatures. Also chemical deposition of such as titanium diboride from vapor has been attempted. Coatings prepared by these methods have shown poor stability in FeS.sub.2 and LiCl-KCl at temperatures around 500.degree. C. Often micrometer size cracks resulted from the extreme thermal cycles required in applying the boron coating to the substrate metal.